The invention concerns an apparatus for the drying of moist particulate material in superheated steam in a closed vessel which is configured like a body of rotation. The vessel has a lower cylindrical part which is connected via a conical transition piece to an upper cylindrical part with a greater diameter. The vessel has a central part in which there is a heat exchanger, under which there is disposed a transport element for steam, e.g. in the form of a blower such as a centrifugal blower. The vessel comprises a number of upwardly open, elongated and substantially vertical processing cells which are placed around the central part with the heat exchanger. The last of these processing cells has a closed bottom and is a discharge cell, while the remainder have a bottom through which steam can permeate. The processing cells, which lie at the side of one another, are open at the top opposite a common transfer zone, and at the bottom stand in mutual connection through openings at the lower ends of the cells. The particulate material is introduced into the first of the processing cells and is dried during its passage through the processing cells by the superheated steam, which by the transport element for steam from the heat exchanger is blown up through the steam-permeable bottoms, in that the particulate material can pass from one processing cell to the next through the above-mentioned openings.
The material to be dried is led into the first of the processing cells, where it is brought into a swirling movement by the steam which flows up through the cell's steam-permeable bottom. The heaviest particles pass from the one processing cell to the next through openings at the bottom. The lighter particles are blown up into the conical part which is similarly divided into cells. These are furthermore divided by inclined plates which form tapered cone surfaces. Opposite the lowermost parts of the cone surfaces there are openings between the processing cells to which the material is fed by guide rails placed on the cone surfaces. Above the cells there is a common zone where material is also fed forward to-wards the discharge cell. Unlike the remaining cells, no steam flows up through the bottom of the discharge cell. Therefore, all of the material which reaches forward to this cell falls down into the bottom, from which it is led away.
Apparatus of this type is known, for example from EP Patent Publication no. 153 704 (corresponding to U.S. Pat. No. 4,813,155), EP Patent Publication no. 537 262 (corresponding to U.S. Pat. No. 5,357,686) and EP Patent Publication no. 537 263 (corresponding to U.S. Pat. No. 5,289,643).
The use of the apparatus for the drying of sugar beet pulp (pulp) is discussed in an article by Arne Sloth Jensen in International Sugar Journal, November 1992, Vol. 94, No. 1127. The dried beet pulp is normally used as cattle feed. The apparatus is especially applicable precisely within the sugar industry. Within this as well as other industries, the use of the apparatus enables the drying to take place with-out aeration of the product and without loading the environment, in that the drying is effected in a closed container, in this case under pressure. Consequently, nothing is discharged out into the atmosphere, unlike the conventional drum dryers which can be smelt approx. 20 km away. The water which is removed from the moist product leaves the drier as steam. This steam contains all of that energy which is used for the drying, and it can be used in the factory as process steam. A normal sugar factory hereby saves between 50 and 120 tons of fuel oil per day, or a corresponding amount of other fuel. Moreover, the process makes it possible for a sugar factory to keep the whole of the production running with Bio-fuel by burning the dried pulp from the process, said pulp in dried form containing more energy than the sugar factory is required to use. In such a case, an approximately three times greater amount of fuel is saved.
The known apparatus can also be used for the drying of wood chips or other moist fuels, whereby the overall energy yield is increased.
However, the known apparatus has proved unable to dry a product satisfactorily if the product involved is one which contains too many coarse particles which require a relatively long drying time. If a beet pulp is involved one can, in the cases where the percentage of coarse particles is not too great, solve the problem by over-drying the remaining particles so that the material on average attains the correct content of dry substance. However, this can be done only by raising the temperature of the circulating steam, whereby a considerable part of the capacity of the apparatus (approx. 10%-40%) is lost. At the same time herewith, the quality of the product is reduced.
In those case where the percentage of coarse particles is particularly high, the situation can arise in which the product can simply not be dried to the drystuff content (approx. 90%) which is required in order for the final product to be stored. In such cases the hitherto-known drying technology is unusable. Such a situation can arise, for example, if it is desired to dry beet pulp from beets which have been frozen.